Such markings are used to identify bank notes and other financial products such as securities, checks, admission tickets and similar, as well as for product and brand protection. Such markings are also being used increasingly in security technology. They are used there for example to mark access IDs.
In accordance with the state of technology it is generally known that colors can be determined on surfaces with the aid of spectrometers. From U.S. Pat. No. 5,369,481 as well as U.S. Pat. No. 4,968,143 devices for the measurement of colors are known which use the measuring principle of the so-called Ulbricht's sphere. With this the surface is illuminated with white light at various angles. The diffused light is acquired at a fixed angle and spectrally analyzed. The known devices are expensive. They are not suitable for measurement of colors which change depending on the angle of observation.
From U.S. Pat. No. 5,042,893 a device is known for spectral analysis. With this the reflected light from a sample is coupled into a light-conducting fiber and conducted to a spectrometer. There it is split spectrally and the intensity of the spectrally split light is measured with a photo diode bank. The device is particularly expensive due to the required light-conducting fiber optics. It is not immediately suitable for measuring of colors which change depending on the angle of observation.
U.S. Pat. No. 6,285,452 describes a device for measuring colors during which the surface of a sample is irradiated without using light-conducting fiber optics. The light thrown back by the sample is analyzed spectrally. This device also requires expensive spectral-splitting components. It is also not suitable for the measurement of colors which change depending on the angle of observation.
From WO 02/18155 A2 a forgery-proof marking is known. The marking has colors which change depending on the angle of observation. These so-called tilt-angle colors are created by a cluster layer positioned at a specified distance from a metal layer. Tilt-angle colors created like this have a particularly distinct and characteristic spectrum.
Moreover from DE 44 34 168 A1 or DE 199 62 779 A1 for example, devices are known which determine the quality of color surfaces. Such devices are used to determine the color and gloss of the surface. However such devices are not suitable for the determination of tilt-angle colors.